Volume 24, number 3, December 2010

Agri-food industry: High energy savings potential

Economic activities and productivity have fluctuated a lot in the last ten years. Energy efficiency projects are one way of maintaining productivity or reducing economic upheavals over time.

For example, in Québec in 2008, international exports of bio-food products rose to $4.9 billion, which represents 12% of all Canadian bio-food exports. This is therefore an important economic activity sector where the potential for ecoenergy projects is high.

Also, Gaz Métro offers energy efficiency programs, including the Feasibility Studies and Implementation Incentives Programs, which support carrying out such projects.

Whether it is a food production or a processing plant, the energy needs are often very significant. Natural gas is used to meet various thermal requirements, including the following:

  • Steam production
  • Hot water production
  • Cooking furnaces
  • Drying
  • Heating
  • Ventilation

Natural gas consumption can be optimized for each of these applications, either directly or indirectly.

Effluent fluid heat recovery

In a typical food processing plant, various fluids circulate, are heated, cooled, then sent down the drain or perhaps stored. The residual heat can be recovered to preheat other fluids. For example, the hot water used for cooking canned foods can be cooled by preheating the washing water. Since these activities are not necessarily simultaneous, surge tanks are essential. In the cheese industry, when the whey is the heat can be recovered to preheat the raw milk, for example, or to preheat the washing water. If the heat recovered from effluents matches the needs for heating, it can be transferred to preheating the fresh air through the interplay of exchangers and pumps. The costs of installing such a measure and the potential savings will determine the cost-effectiveness of such projects. Several Gaz Métro customers have implemented this measure; Table 1 summarizes some projects for three customers.

Heat recovery from a liquid fluid – table 1

Applications Savings (m3) Payback period (years)
Process fluid recovery 4,513 4.3
Process fluid recovery 146,644 2.05
Process fluid recovery for heating 115,649 1.8

Flue gas heat recovery

One or several steam boilers are used for cooking, sterilizing, cleaning, production, heating and ventilation. The energy contained in the vented gases is often of a high quality; that is, it has a high heat recovery potential since the temperature is high. There are two recovery methods: the heat economizer and the heat reclaimer. The economizer is an indirect exchanger that transfers just the sensible heat from the gases to a fluid (supply water for the boiler, for example). The condensing heat reclaimer can transfer both the sensible and the latent heat from the gases to a fluid. That is, the steam in the gases condenses and releases energy in the form of latent heat.

The economizer structure (dimen­sions and materials) is such that the supply water has to enter the exchanger at a sufficiently high temperature to avoid condensation forming on the products of combustion. Since food processing plants often need steam all year round, it may be cost-effective to install such equipment, even with a 100 HP boiler. It all depends on the operating conditions.

There are two types of condensing heat reclaimers: direct contact and indirect contact. The structure of an indirect-contact device allows the steam contained in the gases to condense. Thus, the temperature of the water entering the exchanger can be lower. The direct contact reclaimer is an exchanger in which the gases and the fluid to be heated are in direct contact; that is, there are no tubes or fins between the two. These devices are more expensive than an economizer, but the savings are much more significant. Table 2 illustrates two flue gas heat recovery projects at two different customers.

Heat recovery from a gaseous fluid – table 2

Applications Savings (m3) Payback period (years)
Heat recovery and ventilation controls 49,272 3.05
Heat recovery and ventilation controls 180,500 4.1

Efficient heating and ventilation

For heating and ventilation, the measures currently used are appropriate. For example, the advantages of infrared heating no longer need to be proven! In production areas, warehouses, loading docks, etc., the applications are varied and success stories abound. In ventilation, the optimization of control systems, heat recovery, and preheating fresh air with the help of solar walls are just some of the most popular measures. In an energy saving project at a company specializing in food production, the greatest challenge lies in the choice of the best combinations to make the plant efficient with cost-effective projects.

Process integration in the agri-food industry

Optimizing the thermal balance in a food production or processing plant is far from simple. The process integration approach presented briefly in last September’s Informa-TECH may prove to be a useful in helping make the most cost-effective choices. In fact, with the help of this methodology and the software designed by the CanmetENERGY team, all the systems can be simulated: the boilers, refrigeration, compression and the various exchangers. Choosing and prioritizing projects is based on the results of the process integration. In fact, improving energy consumption has a direct impact on a plant’s productivity gains and fully deserves to be accorded time and money.

Marie-Joëlle Lainé, Eng.